Light emitting and light detecting semiconductor device for interfacing with an optical fiber

ABSTRACT

A light emitting and light detecting semiconductor device having a light emitting zone and a light detecting zone adaptable for concurrently interfacing with an optical fiber.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of semiconductor devices and, moreparticularly, to a semiconductor device having a light emitting zone anda light detecting zone adaptable for concurrently interfacing with anoptical fiber.

Semiconductor devices such as the present invention are useful incommunicating between two points over an optical fiber in lieu ofconventional cables and electronic transmission. The advantages of suchcommunication include the electrical isolation commonly obtained withoptical couplers as well as an immunity to noise picked up andtransmitted by conventional electronic cables.

2. Description of the Prior Art

Semiconductor devices having both a light emitting zone and a lightdetecting zone do exist. However, I am unaware of any device such as thepresent invention which is capable of directly interfacing a singleoptical fiber with separate light emitting and light detecting zones.Thus, to the best of my knowledge, the prior art is comprised of deviceshaving only single zones adaptable for interfacing with an opticalfiber. In such devices, the zones perform either both light emitting andlight detecting functions, or perform only one of these functions. Thepresent invention has significant advantages over such prior art.

An advantage of the present invention over using one device having alight emitting zone and another device having a light detecting zone isthat one does not require a fiber optic assembly having a Y branchleading to separate semiconductor devices. In such a fiber opticassembly, three optical fibers are joined at a junction, thus forming aY branch having three legs. A first leg goes to a light emitting device,a second legs goes to a light detecting device, and a third leg is usedfor bidirectional data transmission. By eliminating the Y branch throughusing a single optical fiber to interface with both the light emittingzone and the light detecting zone, the present invention improvesoptical transmission since there is no Y branch in which optical lossesoccur.

In addition, the present invention has advantages over a device having asingle zone used for both light emitting and light detecting functions.With the present invention, the operating characteristics of both thelight emitting zone and the light detecting zone can be optimized fortheir respective purposes, while a device having a single zone for bothlight emission and light detection is generally not optimized for bothfunctions. Thus, a single zone device generally performs at least one ofits functions less than optimally, especially when normal biasingvoltages are applied.

As a further advantage over single zone devices, the present inventionprovides separate electrical contacts for the emitter zone and thedetector zone. As a result, there is no need to multiplex one electricalcontact and share it for both light emitting and light detecting as isnecessary if a single zone were used for both functions. Thus, thepresent invention may allow use of more simple circuitry than ispossible with single zone devices performing both functions.

SUMMARY OF THE INVENTION

The present invention is a light emitting and light detectingsemiconductor device having a light emitting zone and a light detectingzone adaptable for concurrently interfacing with an optical fiber. Thelight emitting and light detecting zones extend into a primary regionfrom a first surface of the primary region. The primary region is of afirst conductivity type, the zones being of a second conductivity type.A separation region between the zones separates the zones from eachother.

A first preferred embodiment of the present invention includes a groovebetween the zones. In a second preferred embodiment, the separationregion includes a separation material different from the material of theprimary region extending from the first surface to a depth greater thanthe depth of either zone. In a third preferred embodiment of the presentinvention, an optical fiber is interfaced with the device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the top view of a preferred embodiment of the presentinvention.

FIG. 2 is a cross section of FIG. 1.

FIG. 3 is an embodiment cross section similar to FIG. 2, together withthe addition of an optical fiber and a positioning apparatus forpositioning the optical fiber.

FIG. 4 is a circuit for using the device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Structure

Referring to FIGS. 1 and 2, an embodiment 8 of the present inventionincludes a gallium arsenide or gallium arsenide phosphide primary region9 which may be comprised of both a substrate 10 and an epitaxial layer12 grown on substrate 10, or entirely of substrate 10, as is shown inFIG. 3. Epitaxial layer 12 is grown on substrate 10 in order to create aregion having relatively well known composition. Typically, substrate 10is approximately 100 to 200 microns thick, and epitaxial layer 12 isapproximately 10 to 50 microns thick, a nominal thickness of epitaxiallayer 12 being 20 microns.

Light detecting zone 13 and light emitting zone 14 extend into primaryregion 9 from a first surface 15 of primary region 9, zones 13 and 14being separated from each other, typically by a distance of 5 to 10microns. Typically, zones 13 and 14 are formed of P type material, withprimary region 9 being of an N type material. For N type galliumarsenide, the dopant is typically silicon or tellurium; for P typegallium arsenide, germanium is typically used.

A second surface 18 oppositely facing first surface 15 of primary region9 is typically covered with a metalization layer such as gold, thusforming a primary region contact 11.

Light detecting zone 13 should be of such a nature that whenever ajunction 16 between light detecting zone 13 and primary region 9 isreverse biased, incoming radiation is absorbed substantially within anelectron diffusion length of junction 16. This can be accomplished byeither making light detecting zone 13 transmissive to incoming radiationsuch that absorption will occur below junction 16 or by making lightdetecting zone 13 shallow compared to the electron diffusion length ofapproximately 5 microns. In the latter case, a typical depth of lightdetecting zone 13 is on the order of 3 microns.

Light detecting zone 13 typically has a doping level on the order of10¹⁶ atoms per cubic centimeter, a level which is generally less thanthe doping level of light emitting zone 14. The doping level of lightdetecting zone 13 is designed to be low enough to allow a wide depletionregion between P type zone 13 and N type primary region 9 for absorptionof radiation.

Light emitting zone 14 should be of such a nature that whenever ajunction 17 between light emitting zone 14 and primary region 9 isforward biased, radiation generated at junction 17 is emittedsubstantially at the level of first surface 15 rather than beingsubstantially absorbed within the zone. In order to accomplish this,light emitting zone 14 is typically on the order of 5 microns deep.

Light emitting zone 14 has a typical doping level on the order of 10¹⁸atoms per cubic centimeter in order to achieve high conductivity in thezone and to achieve fast response.

First surface 15 should generally be at least partially covered with adielectric layer 20 such as silicon dioxide. A primary purpose ofdielectric layer 20 is to provide electrical insulation between primaryregion 9 and any portion of the zone contacts (discussed below) whichprotrude beyond the area of the zones. In addition, the dielectric layermay help prevent deterioration through contamination and thus may helpto prevent leaking of junctions 16 and 17.

Dielectric layer 20 need not cover light detecting zone 13 and lightemitting zone 14 any more than is necessary to provide appropriateelectrical insulation from the primary region for the zone contactsdiscussed below. Thus, FIG. 3 shows a portion of the area above zones 13and 14 being free of any dielectric layer 20.

If light detecting zone 13 and light emitting zone 14 are substantiallycovered with dielectric layer 20 as shown in FIG. 2, it is necessarythat at least an appropriate portion of dielectric 20 be lighttransmissive in order to permit incoming light to be received by lightdetecting zone 13 and to permit light emitting zone 14 to emit light.Such a layer over zones 13 and 14, if of appropriate thickness(typically a quarter of a wavelength), can help reduce surfacereflection of incoming radiation. In addition, such a layer may help toprevent deterioriation. In FIG. 2, a central portion of dielectric layer20 is crosshatched as being a transparent material, the remainingportion being crosshatched as an electrical insulator.

In order to permit electrical connections to be made to light detectingzone 13 and light emitting zone 14, a separate contact is typicallyconnected to each zone. A light detecting zone contact 21 typicallycovers a portion of dielectric layer 20 and extends into light detectingzone 13 with at least one protrusion 22 having a first surface areawhich is small in comparison to the first surface area of lightdetecting zone 13. Similarly, a light emitting zone contact 23 typicallycovers at least a portion of dielectric layer 20 and extends into lightemitting zone 14 with at least one protrusion 24 having a first surfacearea which is small in comparison to the first surface area of lightemitting zone 14.

Light detecting zone 13, junction 16, primary region 9 and contacts 21and 11 generally comprise a photodiode 27. Light emitting zone 14,junction 17, primary region 9, and contacts 23 and 11 generally comprisea light emitting diode 28.

Optical isolation between light detecting zone 13 and light emittingzone 14 may be provided either physically by a barrier between the zonesor by electronic circuitry permitting only non-simultaneous operation oflight detecting zone 13 and light emitting zone 14. If a barrier isdesired, it can comprise a groove 25 etched between the two zones andmay include a material 26 different from the material of primary region9 at least partially filling groove 25. It should perhaps be noted thatthe double crosshatching used to designate material 26 in FIG. 2 is onlydesignating a material different from primary region 9 and is not meantto indicate any particular material or other characteristic.

As previously indicated, FIG. 3 illustrates an embodiment cross sectionsimilar to FIG. 2 except that primary region 9 comprises substrate 10and does not include epitaxial layer 12. In addition, a portion of thearea above zones 13 and 14 is shown free of any dielectric layer 20, andgroove 25 is shown opening to first surface 15 and being free of anymaterial 26. FIG. 3 also illustrates an optical fiber 29 positioned witha port 30 in proximity to light detecting zone 13 and light emittingzone 14. Also shown is positioning means 31 for positioning opticalfiber 29 and port 30.

Operation

In operation, light detecting zone 13 generates a current whenever lightis received by zone 13 while junction 16 is reversed biased. Reversebiasing is accomplished by applying a negative potential to lightdetecting zone contact 21 with primary region contact 11 connected to areference potential such as ground. Light emitting zone 14 emits lightwhenever junction 17 is forward biased by applying a positive potentialto light emitting zone contact 23 with primary region contact 11connected to the reference potential.

A circuit 39 for operating the device is shown in FIG. 4. Circuit 39comprises an input terminal 40 adapted to receive two-state logicsignals having first and second signal levels. Input terminal 40 isconnected to the input of a logic inverter 41 for converting logic highsto logic lows and vice versa. An output of inverter 41 is connected to abase 42 of a transistor 43 which acts as a control means. A collector 44of transistor 43 is connected to a positive voltage terminal 45 througha resistor 46 as well as to light emitting zone contact 23 of lightemitting diode 28. An emitter 48 of transistor 43 is connected to areference potential 49. Primary region contact 11 of light emittingdiode 28 is connected to reference potential 49 as well as to primaryregion contact 11 of photodiode 27. Light detecting zone contact 21 ofphotodiode 27 is connected to a negative voltage terminal 47 through aresistor 50 as well as to an input 51 of an amplifier 52. An output 53of amplifier 52 is connected to an output terminal 54.

When utilizing the present invention, one may select whether to havelight emitting diode 28 on either when data received at input terminal40 is high or when it is low. If it is desired to have light emittingdiode 28 on whenever data received at input terminal 40 is high,inverter 41 should be included in circuit 39 as previously described andshown in FIG. 4. If, on the other hand, it is desired that lightemitting diode 28 be on whenever data received at input terminal 40 islow, inverter 41 should be eliminated from circuit 39, and inputterminal 40 should be connected directly to base 42 of transistor 43.

In operating circuit 39 either with or without inverter 41, wheneverdata received at base 42 of transistor 43 is high transistor 43 turns onand acts as a switch to conduct the current derived from positivevoltage terminal 45 through emitter 48 to reference potential 49. Thus,when transistor 43 is turned on by a high signal level, no voltage isreceived by light emitting diode 28, thus causing light emitting diode28 to be off. Whenever the signal level at base 42 of transistor 43 islow (and is thereby below a predetermined threshold) transistor 43remains off, thus transmitting no current to reference potential 49 andpermitting light emitting diode 28 to be on, diode 28 being forwardbiased by the voltage derived from positive voltage terminal 45.

Photodiode 27 is reverse biased by a negative voltage derived fromnegative voltage terminal 47. Whenever light is received by photodiode.[.18.]. .Iadd.27 .Iaddend.while it is reversed biased, the change inoutput from photodiode 27 is amplified by amplifier 52 and detected atoutput terminal 54.

The embodiments of the invention in which an exclusive property or rightis claimed are defined as follows:
 1. A light emitting and lightdetecting semiconductor device having a light emitting zone and a lightdetecting zone adaptable for concurrently interfacing with an opticalfiber, comprising:a primary region of a first conductivity type; thelight detecting and light emitting zones being of a second conductivitytype extending into the primary region from a first surface of theprimary region, the zones being separated from each other by aseparation region, a junction being formed at an interface between eachzone and the primary region, the zones having a depth measured from thefirst surface; and a separation material different from the material ofthe primary region, the separation material extending into at least aportion of the separation region from the first surface to a depthgreater than the depth of either zone, the separation material providingsubstantial optical isolation between the two zones.
 2. A light emittingand light detecting semiconductor device having a light emitting zoneand a light detecting zone adaptable for concurrently interfacing withan optical fiber, comprising:a primary region of a first conductivitytype; the light detecting and light emitting zones being of a secondconductivity type extending into the primary region from a first surfaceof the primary region, the zones being separated from each other by aseparation region, a junction being formed at an interface between eachzone and the primary region, the zones having a depth measured from thefirst surface; and the device having a groove opening to the firstsurface between the two zones .Iadd.as part of a structure for providingsubstantial optical isolation between the two zones, the groove being atleast partially filled with a material different from the material ofthe primary region in order to substantially optically isolate the twozones from each other.Iaddend..
 3. A device as in claim 2 wherein thegroove extends to a depth greater than the depth of either zone. .[.4. Asemiconductor device as in claim 2 wherein the groove is at leastpartially filled with a material different from the material of theprimary region in order to substantially optically isolate the two zonesfrom each other..].
 5. A light emitting and light detectingsemiconductor device having a light emitting zone and a light detectingzone adaptable for concurrently interfacing with an optical fiber,comprising:a primary region of a first conductivity type; the lightdetecting and light emitting zones being of a second conductivity typeextending into the primary region from a first surface of the primaryregion, the zones being separated from each other by a separationregion, a junction being formed at an interface between each zone andthe primary region, the zones having a depth measured from the firstsurface; .[.and.]..Iadd. a separation material different from thematerial of the primary region, the separation material extending intoat least a portion of the separation region from the first surface to adepth greater than the depth of either zone, the separation materialproviding substantial optical isolation between the two zones; and.Iaddend. the optical fiber having a port located in proximity to thelight detecting and light emitting zones such that at least a portion ofany light at the port is received by the light detecting zone and atleast a portion of any light emitted by the light emitting zone isreceived by the port for transmission over the optical fiber.
 6. Asemiconductor device as in claims 1, 2, or 5 wherein the light detectingzone is of such a nature that, should the junction between the lightdetecting zone and the primary region be reverse biased, incomingradiation is absorbed substantially within an electron diffusion lengthof the junction between the light detecting zone and the primary region,and wherein the light emitting zone is of such a nature that, should thejunction between the light emitting zone and the primary region beforward biased, radiation generated at the junction between the lightemitting zone and the primary region is emitted substantially at thefirst surface level of the zone rather than being substantially absorbedwithin the zone.
 7. A semiconductor device as in claims 1, 2, or 5wherein the depth of the light detecting zone is on the order of threemicrons and wherein the depth of the light emitting zone is on the orderof five microns.
 8. A semiconductor device as in claims 1, 2, or 5wherein the primary region and the light emitting zone each have adoping level which is approximately equal to a first doping level andwherein the light detecting zone has a second doping level, the firstdoping level being approximately two orders of magnitude greater thanthe second doping level.
 9. A semiconductor device as in claims 1, 2, or5 wherein the primary region is formed of an N type material and whereinboth the light detecting and light emitting zones are formed of a P typematerial.
 10. A semiconductor device as in claims 1, 2, or 5 wherein theprimary region and the light detecting and light emitting zones areformed of a material selected from the group consisting of galliumarsenide and gallium arsenide phosphide.
 11. A semiconductor device asin claims 1, 2, or 5 wherein the primary region further comprises anexpitaxial layer adjacent the first surface of the device, the thicknessof the layer being in the range of approximately ten to fifty microns.12. A semiconductor device as in claims 1, 2, or 5 further comprising:alight detecting zone contact connected to the light detecting zone; alight emitting zone contact connected to the light emitting zone; and aprimary region contact connected to the primary region and adapted forconnecting the primary region to a reference potential.
 13. Asemiconductor device as in claims 1, 2, or 5 further comprising:adielectric layer covering at least a portion of the first surface; alight detecting zone contact covering at least a portion of thedielectric layer and extending into the light detecting zone with aprotrusion having a first surface area which is small in comparison withthe first surface area of the light detecting zone; a light emittingzone contact covering at least a portion of the dielectric layer andextending into the light emitting zone with a protrusion having a firstsurface area which is small in comparison to the first surface area ofthe light emitting zone.
 14. A semiconductor device as in claims 1, 2,or 5 wherein the primary region has a second surface and wherein aprimary region contact is connected to the primary region and covers atleast a portion of the second surface, the primary region contact beingadapted for connecting the primary region to a reference potential. 15.A semiconductor device as in claims 1, 2, or 5 wherein the primaryregion has a second surface oppositely facing the first surface andwherein a primary region contact is connected to the primary region andcovers at least a portion of the second surface, the primary regioncontact being adapted for connecting the primary region to a referencepotential.
 16. A semiconductor device as in claims 1, 2, or 5 whereinthe zones are separated from each other by a distance approximately fiveto ten microns.
 7. A device as in claims 1 or 2 further comprising theoptical fiber having a port located in proximity to the light detectingand light emitting zones such that at least a portion of any light atthe port is received by the light detecting zone and at least a portionof any light emitted by the light emitting zone is received by the portfor transmission over the optical fiber.
 18. A device as in claims 1, 2,or 5 further comprising positioning means connected to the device, thepositioning means being adapted for positioning a port of an opticalfiber in proximity to the light detecting and light emitting zones. 19.A device as in claims 1, 2, or 5 further including an electricalcircuit, comprising:an input terminal for receiving input signals havingfirst and second levels; means comprising control means connectedbetween the input terminal and the light emitting zone for permittingthe application of a positive voltage to the light emitting zonewhenever the first signal level is present at the input terminal; apositive voltage terminal adapted to receive the positive voltage; meansconnecting the positive voltage terminal to the control means; anegative voltage terminal adapted to receive a negative voltage; meansconnecting the negative voltage terminal to the light detecting zone; anoutput terminal; and means connecting the output terminal to the lightdetecting zone for providing a signal at the output terminal wheneverlight is received by the light detecting zone while the junction betweenthe light detecting zone and the primary region is reverse biased.
 20. Adevice as in claims 1, 2, or 5 further including an electrical circuit,comprising:an input terminal for receiving input signals having firstand second signal levels; a positive voltage terminal adapted to receivea positive voltage; a negative voltage terminal adapted to receive anegative voltage; means connecting the negative voltage terminal to thelight detecting zone; inverter means for converting logic highs to logiclows and vice versa, the inverter means having an input connected to theinput terminal, the inverter means also having an output; control meanscomprising a transistor with a base connected to the output of theinverter means, an emitter adapted for connection to a referencepotential, and a collector; means connecting the collector of thetransistor to the light emitting zone; means connecting the collector ofthe transistor to the positive voltage terminal; an amplifier having aninput and an output; means connecting the input of the amplifier to thelight detecting zone; an output terminal; means connecting the outputterminal to the output of the amplifier; and means adaptable forconnecting the primary region to the reference potential.